The invention relates to cables subject to fluid flow. In particular, the invention relates to a cable subject to fluid flow having a polygonal cross section and a helical outer surface for reduction of strumming or transverse vibration caused by fluid flowing past the cable.
It has been well known for some time that fluids such as air or water flowing in a transverse direction past a stretched body, for instance a cable or a wire having a circular transverse cross section, generate lift and drag forces, as well as vortices in the fluid adjacent to and downstream from the cable. In some instances, the vortices may be simultaneously generated and shed at portions of the cable displaced 90.degree. from the direction of fluid flow, thereby leaving the lift forces on the cable in balance transverse to the direction of fluid flow. It has also been found, however, that vortices may be formed and shed alternatively on opposite sides of the cable. The vortices are aligned parallel to the cable in columns known as von Karman vortex streets. When the vortex streets are formed and shed alternately on respective sides of the cable, the lift forces are unbalanced, and a resulting periodic transverse alternating force is applied to the cable. If the shedding frequency of the alternating vortices approaches or is equal to a natural frequency of the cable, the cable will be driven resonantly into large transverse vibrations. The resonant vibrations, known as strumming, drastically accelerate deterioration of the cable due to structural fatigue. For instance, a typical cable may have a life of one million to ten million flexures. Such a cable may be driven transversely at frequencies as high as 100 Hertz by fluid flowing past it. Since the resonant vibration leads to high stress concentrations at the nodal points on the cable, a cable resonating at 100 Hertz could well break within the space of a few days.
In order to lengthen cable life by reducing cable failure due to resonant vibration or strumming, a number of solutions have been attempted which affect the formation of the von Karman vortex streets. One solution teaches modifying a normally smooth circularly cylindrical surface of the cable by roughening the surface. Typically, splitter plates extend outwardly from the cable at random points in an attempt to alter the separation positions of the vortices from the outer wall of the cable and reduce the resonance effects along the length of the cable. Splitter plates, however, are relatively expensive and not very effective.
A number of fairings have also been used in association with the cables. One such fairing, referred to as a haired fairing, employs heavy monofilament urethane strands having a length approximately three to four times the diameter of the cable and extending outwardly therefrom. Again, it is relatively expensive to form the strands on the cable jacket. Although the strands may displace the vortex streets downstream from the cable along the flow path of the current, they do not eliminate the production of the oscillatory force which resonantly drives the cable.
A still further solution which has been attempted by those skilled in the art is the use of a NACA (National Advisory Committee on Aeronautics) fairing of the type similar to an airfoil but having symmetrical surfaces. NACA fairings, however, must be free to pivot about the cable as the direction of the current or fluid flow changes. As a result, the fairings are very expensive, although they do suppress the production of vortices and concomitant strumming.
Workers in the art have also employed helically wound strakes to reduce vibration and strumming in circular cylindrical structural members such as ocean structures, drill pipes and riser casings. Each of the strakes consists of a strain cable having a semi-rigid, polyvinyl chloride jacket with a three pointed star cross-section surrounding it. The strakes are spaced 120.degree. apart on the structural member to be protected and are wrapped around it in a triple helix. They may be purchased from Fathom Oceanology Ltd., Mississauga, Ontario, Canada, which identifies them with the trademark Starstrake. The Starstrake system suffers from the disadvantage that it requires a rigid column as a central structure. The column is placed in compression by the strakes. Clearly, the strakes could not be employed with a central member which could not withstand a compressional load, such as a cable.
What is needed is a new cable form which reduces or eliminates the production of resonant transverse forces on the cable.